The present invention relates to an electrostatic proximity sensor, and more particularly to an electrostatic capacitance-type proximity sensor having improved noise characteristics. The proximity sensor has an improved range of detection due to the improved noise characteristics of the device. The proximity sensor is used to detect the presence of objects, including human or animal bodies, within a given range.
Conventional electrostatic capacity proximity sensors function on the basis of determining a change in capacity of a detection electrode when objects are near the detection electrode. A proximity sensor of this type is disclosed in Japanese Laid-Open Utility Model Publication No. Sho. 63-36246. The proximity sensor includes a pulse-generating circuit having an output applied to two delay circuits, each having an integrator configuration. A second one of the delay circuits uses a fixed capacitor and serves to create a reference delay. A first one of the delay circuits has a capacitance determined by an electrostatic capacity of the detection electrode. The capacitance of the detection electrode increases when objects are placed proximate the detection electrode. The increase in capacity increases a delay in the first delay circuit. Outputs of the first and second delay circuits are each applied to a limiting amplifier, or Schmitt trigger, which shapes the output into digital pulses. The outputs of the limiting amplifiers are applied to a phase detection circuit which determines the sequence of arrival of rising edges of the respective outputs.
Normally, the second delay circuit serves as a reference and has a longer delay than the first delay circuit when no objects are near the detection electrode. Thus, in a state where no objects are proximate the detection electrode, rising edges of pulses from the first delay circuit arrive at the phase detection before those of the second delay circuit. When an object is disposed near the detection electrode, the delay of the first delay circuit increases such that the sequence of arrival of the rising edges at the phase detector is reversed. When the reversal occurs, the phase detector outputs a high signal indicating that an object is within range of the detection electrode.
Since the first and second delay circuits are of a resistor-capacitor integrator-type configuration, their outputs are based on an exponential function of time. Therefore, the slope of the output curves of the first and second delay circuits is dependent upon a time relative to the beginning of charging of the capacitances of the respective delay circuits. The sensitivity of the proximity sensor is determined in part by the effect of noise disturbance on the output of the delay circuits. Since the outputs of the delay circuits are not linear, and are exponential in nature with respect to time, a given noise spike can produce errors in delays varying with the timing of the noise spike. Therefore, a given noise spike may produce lesser or greater errors based on its timing with respect to the beginning of the integration period. For instance, if the noise spike occurs shortly after the beginning of an integration period, the slope of the output signal is relatively steep and the error caused by the noise spike is minimal. However, should the noise spike occur toward the end of the integration period, the slope of the output at that point is minimal and the recovery from the noise spike becomes protracted. Thus, in order to determine the accuracy and range of the proximity sensor, one must consider the possibility of the noise spike occurring at a time which maximizes the influence of the noise spike on the delay of the delay circuit.
The range of detection of the proximity sensor is determined by the smallest capacitance change that can be reliably detected. Objects distant from the detection electrode produce smaller changes in capacitance than objects near the detection electrode. In the stand-by state, the delay of the reference delay circuit, that is the second delay circuit, is greater than that of the first delay circuit by an amount .DELTA.T sufficient to prevent noise from producing false triggering. Therefore, a change in capacitance of the detection electrode must be sufficient to overcome the delay .DELTA.T. Thus, a reduction in the required level of the delay .DELTA.T would increase the detection range of the proximity sensor. Accordingly, it would be desirable to have a constant lower range of error due to noise spikes than the prior art throughout the entire integration period. So long as the error level is less than the error level towards the end of the integration period in conventional circuits, the detection range of the proximity sensor will be improved.
As described above, the sensitivity of a proximity sensor is determined by the minimal phase difference which may be reliably detected. The minimal phase difference which can be detected is, in part, determined by the stability of the delay circuits used in the proximity sensor. Where the error created by a given noise spike depends on the timing of its arrival, the worst case timing will dictate the reliability and range of the proximity sensor. Since errors in delays directly affect errors in phase difference measurement, it would be a welcomed innovation to provide a proximity sensor which has errors due to noise spikes which are independent of the timing of the noise spikes and less than longer delays associated with an equivalent noise spike in a conventional proximity sensor.